1. Field of the Invention
The present invention relates to an optical information recording medium such as optical discs, more specifically, relates to an optical information recording medium having two recording layers, and a recording and reproducing apparatus for the optical information recording medium.
2. Description of the Related Art
Compact discs (CD) have become widely used and have established an important position. In addition, not only reproduce-only optical discs but also CD-R/RW (CD Recordable/Rewritable), and DVD±R/RW (Digital Versatile Disc±Recordable/Rewritable) have remarkably become popular as write once read many (WORM) optical discs and rewritable optical discs which enable recording information. Recently, there has been a blossoming of studies and developments for higher-density optical discs.
In the case of CD-R (Read Only Compact Disc-Recordable) and DVD±R (Read Only Digital Versatile Disc±Recordable), information is recorded and reproduced by reflectance variations induced by deformation of dyes. In the case of rewritable optical discs, recording information signals are detected by utilizing reflectance difference and changes of phase difference induced by refractive index difference between amorphous condition and crystalline condition.
To increase storage capacity of one optical disc, a system having a plurality of data layers has been proposed. An optical disc having two or more data layers enables gaining access to various layers by changing the focal point of lens.
For example, U.S. Pat. No. 5,202,875 discloses an optical disc drive system having a plurality of data layers, and the optical disc used for the system comprises a plurality of substrates provided with data layers each of which is disposed at intervals with air-gap or a plurality of data layers in solid structures.
In U.S. Pat. No. 4,450,553, the recording medium employs a plurality of data layers in solid structures, and each of these data layers is a CD type data layer.
Each of the optical recording media disclosed in International Publication Nos. WO00/016320, and WO00/023990 has a recording layer which comprises only a dye layer or a phase-change layer, and the recording medium does not employ a laminar structure allowing for compatibility with reflectance, DVD, and the like. A first aspect of the invention described that it is preferred that the information layer preferably comprise at least two or more layers, however, the invention does not actually allow for a two-layered recording layer.
Japanese Patent Application Laid-Open (JP-A) Nos. 2001-084643 and 2001-10709 disclose a recording medium having two recording layers which comprise organic materials, however, the recording medium is not at the practical use level.
As described above, there are various proposals for optical recording media, however, practically, most of the recording media have not been put into practical use and/or do not allow for compatibility within reflectance, and the like.
Japanese Patent Application Laid-Open (JP-A) No. 03-157830 describes that the recording thin layer comprises two or more layers, and the material of the thin layer changes its optical constant by irradiating a laser beam to the recording thin layer, and the detectable change of the optical constant is induced primarily by phase change of a reflected beam or a transmitted beam of the incident beam. However, the invention does not allow for compatibility with ROM (Read Only Memory), and the invention has a laminar structure different from that of the present invention as described in the invention as follows: An optical information recording medium comprises a first transparent layer, a first recording thin layer, a second transparent layer, a second recording thin layer, a third transparent layer, and a reflective layer, each of which has a refractive index different that of a base material, are sequentially disposed on the base material, and the layer thicknesses for the first transparent layer, the first recording thin layer, the second transparent layer, the second recording thin layer, the third transparent layer, and the reflective layer are selected such that the phase of the transmitted beam or the reflected beam of the incident beam can be changed when a recording material is changed.
For a two-layered reproduce-only recording medium, DVD-ROM has been put into practical use, however, a recordable or rewritable two-layered recording medium has not yet been put into practical use.
Japanese Patent Application Laid-Open (JP-A) No. 10-505188 discloses a recording medium having two information layers, and in claim 16 of the invention, it is described that the sample pit of the second substrate or the track pitch of guide grooves is made smaller than those of the first substrate. With respect to the effect of the recording medium, it is described that excellent signal reproducing can be performed to an information layer having a focal depth which is out of the focal depth of optical systems.
However, in the case of the recording medium, two substrates each having a different eccentricity are bonded each other, therefore, the rotation centers of the respective layers are misaligned each other by the bonding, and thus the eccentric condition of the front side substrate viewed from the laser beam irradiation side is reproduced by the record of a drive or a clamp of the recording and reproducing apparatus, and further, the eccentricity of the inner side substrate viewed from the laser beam irradiation side is added to the eccentricity of the substrate on the laser beam irradiation side by the misalignment of the rotation center when these substrates are bonded. Consequently, the eccentricity of the inner side substrate viewed from the information reproducing apparatus such as a drive or the like, in the worst case, has a loading eccentricity of the eccentricity of the front side substrate viewed from the laser beam irradiation side plus the eccentricity of the inner side substrate viewed from the laser beam irradiation side, which leads to failures in tracking, and makes reproducing, and recording/reproducing impossible.